From the viewpoint of global environmental protection, improvement in the fuel economy of automobiles has recently been required to regulate the amount of CO2 emissions. In addition, it is also required to improve safety by focusing on collision characteristics of automobile bodies to ensure the safety of passengers at the time of a collision. Thus, both weight reduction and strengthening of automobile bodies are being actively promoted. To simultaneously achieve such weight reduction and strengthening of automobile bodies, an increase in the strength of a material for members and a reduction in weight by reducing the thickness of sheets to the extent that rigidity is not impaired are said to be effective. Nowadays, high-strength steel sheets are positively used for automotive parts. Use of high-strength steel sheets results in a significant weight reduction effect. Thus, in the motor vehicle industry, for example, there is a trend toward the use of steel sheets as a structural material with a TS of 540 MPa or more.
Many automotive parts made from steel sheets are manufactured by press forming. Regarding the formability of high-strength steel sheets, dimensional accuracy is important in addition to prevention of cracking and wrinkling. In particular, controlling springback is an important problem. Nowadays, new automobiles are developed very efficiently by computer assisted engineering (CAE). So, it is not necessary to make many dies. Furthermore, the input of the characteristics of a steel sheet enables predicting the amount of springback more accurate. Variations in the amount of springback cause problems when parts are connected to each other and thus should be reduced. So, in particular, a high-strength steel sheet having only small variations in strength and excellent uniformity in strength is required.
As a method for reducing variations in strength in a coil, Japanese Unexamined Patent Application Publication No. 4-289125 discloses the following method: In the case of hot-rolling Nb-containing low-manganese steel (Mn: 0.5% or less), a rough-rolled sheet bar is temporarily wound into a coil. Next, the sheet bar is joined to the preceding sheet bar while being unwound, and then continuously finish-rolled to achieve uniformity in the strength of the high-strength hot-rolled steel sheet in a coil. Japanese Unexamined Patent Application Publication No. 2002-322541 discloses a high-strength hot-rolled steel sheet with excellent uniformity in strength, i.e., only small variations in strength, produced by the addition of both Ti and Mo to form very fine precipitates uniformly dispersed therein.
The foregoing, however, have problems. The method described in JP 4-289125 has a problem in which when the sheet is wound into a coil, the sheet is divided. Furthermore, the addition of Nb causes an increase in cost, which is economically disadvantageous. In the steel sheet described in JP 2002-322541, which is a Ti system, it is necessary to add Mo, which is expensive, thus causing an increase in cost. Moreover, in both publications, two-dimensional uniformity in strength in the in-plane directions including both of the width direction and the longitudinal direction of the coil is not taken into consideration. Disadvantageously, even if the coiling temperature is uniformly controlled, variations in the in-plane strength of the coil are inevitably caused by different cooling histories for each position in the wound coil.
In consideration of the above-described situation, it could be helpful to provide a high-strength hot-rolled steel sheet having a tensile strength (TS) of 540 to 780 MPa, only small variations in strength, and excellent uniformity in strength using a general-purpose Ti-containing steel sheet, which is inexpensive, and to provide a method for manufacturing the high-strength hot-rolled steel sheet.